Examples of typical plastics for optical applications include polymethyl methacrylate and polycarbonate. Polymethyl methacrylate is advantageous in terms of transparency, weather resistance, low birefringence, and good moldability and is used in optical lenses such as a CD pick-up lens, optical films such as a retardation film, and optical disk substrates, and the like. However, polymethyl methacrylate has a serious disadvantage of a high water absorption rate. In the case where the water absorption rate is high, in optical lenses, degradation of light-focusing accuracy and a change in the refractive index are caused by swelling and deformation of the lenses. Furthermore, in disk substrates, deformation and curvature are caused by water absorption, resulting in read/write errors.
Polycarbonate is advantageous in terms of transparency, heat resistance, impact resistance, a low water absorption rate, etc. and is considered to be an important optical material. However, polycarbonate is disadvantageous in that the birefringence, which is an important characteristic in the optical system, is high. In the case where the birefringence is high, in optical disks, the polarization direction of reflected light with respect to incident light deviates, resulting in an increase in noise.
Accordingly, materials that can compensate for these disadvantages have been desired.
In general, polyurethanes are excellent in terms of oil resistance and abrasion resistance, but have low heat resistance. However, since physical properties of polyurethanes significantly vary depending on the type of raw materials, conditions for cross-linking, etc., various physical properties that cannot be exhibited by other homopolymers can be adjusted. As a polyurethane that is applied to optical applications, for example, a polyurethane obtained by using an alkylene oxide adduct of 4,4′-(9H-fluorene-9,9-diyl)bis(phenol) as a dial component has been proposed (for example, refer to Patent Literature 1). However, although a resin having a high refractive index can be obtained in this technology, the glass transition temperature of the resin is also increased and thus the resin is poor in practical use.